1. Field of the Invention
The present invention relates to a silicon semiconductor-based optical device, and more particularly, to a silicon semiconductor-based optical device including an optical waveguide having refractive index variation.
2. Description of the Related Art
Due to the rapid development in silicon semiconductor technology, the speed of semiconductor chips such as computer CPUs, DRAMs, and SRAMs is increasing. However, despite this, the signal communication speed between semiconductor chips has not increased accordingly. In order to address this problem, semiconductor chip communication using light has been suggested.
First, connecting semiconductor chips using optical devices formed of compound semiconductors has been studied worldwide. However, high-speed compound semiconductor optical devices are still relatively expensive, and packaging of the compound semiconductor optical devices and silicon semiconductor chips is complex and expensive. To address these problems, silicon optical devices are integrated with silicon semiconductor chips. Hereinafter, an optical modulator will be described as an example of an optical device.
A silicon optical modulator can be formed using various methods. One of the methods is using the resonance/anti-resonance of a Fabry-Perot cavity. This method is described in detail in “Low-Power Consumption Short-Length and High-Modulation-Depth Silicon Electrooptic Modulator,” Journal of Lightwave Technology, Vol. 21, No. 4, pp. 1089-1098, 2003. In this method, the amount of light transmitted between two mirrors of the Fabry-Perot cavity is controlled by adjusting the amount of a current that is applied to a material, for example, silicon. However, in this method, light is reflected at an input end, and thus a portion of the light returns along the path the light came, and the returning light affects the optical communication, thereby adversely affecting communication.
Thus, an optical modulator using a ring resonator has been suggested to address the problem of reflection.
FIG. 1 is a schematic view illustrating a conventional optical modulator using a ring resonator. Hereinafter, the optical modulator using a ring resonator illustrated in FIG. 1 will be referred to as a ring optical modulator. Briefly, the ring optical modulator functions as follows. Input light Pin input to a direct waveguide 10 is transmitted through the direct optical waveguide 10 and a portion of the input light Pin is coupled to a ring optical waveguide 20. The light coupled to the ring optical waveguide 20 circulates through the ring optical waveguide 20 and is coupled to the direct optical waveguide 10 to generate interference with the input light Pin which is being continuously input. The interference is constructive or destructive depending on the wavelength of the input light Pin.
FIG. 2 is a graph illustrating the transmittance characteristic of light when a current is supplied to the ring optical waveguide 20 of the ring optical modulator of FIG. 1 and when a current is not supplied.
Referring to FIG. 2, the transmittance characteristic of light according to the wavelength of the ring optical modulator varies when the refractive index of the ring optical waveguide 20 is varied. Typically, the ring optical waveguide 20 is formed of silicon, and one of the ways to vary the refractive index of silicon is by supplying a current to the silicon. When the current is supplied to the ring optical waveguide 20, the refractive index of the ring optical waveguide 20 is varied, and the wavelength is varied to cause destructive or constructive interference. Accordingly, the input light Pin which is input at a predetermined wavelength, is output as output light Pout while the input light Pin interferes constructively or destructively to the light transmitted to the ring optical waveguide 20 depending on whether a current is supplied to the ring optical waveguide 20 or not.
An optical modulator formed using the above-described characteristic is disclosed in “Micrometer-scale silicon electro-optic modulator” in Nature, vol. 435, 2005, pp. 325-327.
However, in the above-described optical modulator, a current is supplied to the ring optical waveguide using a P-I-N diode, and when a P-I-N diode is used, the modulation speed is limited due to the characteristics of the P-I-N diode.
FIG. 3 is a cross-sectional view illustrating a P-I-N diode used in a ring optical waveguide.
Referring to FIG. 3, the P-I-N diode is formed of an n-type doping region 40, a p-type doping region 50, and an intrinsic region 60. Propagated light A is mostly transmitted through the intrinsic region 60. When the distribution of carriers such as electrons or holes of the intrinsic region 60 is changed, the refractive index of material is varied, and thereby light is modulated in the ring optical modulator as described above. Here, carriers should be supplied and discharged at high speed in the intrinsic region 60, that is, the region through which light is transmitted, for high speed optical modulation; however, the P-I-N diode has a property of which the speed of supply and discharge of carriers is low in the intrinsic region 60.
In detail, from the perspective of supply of carriers, the volume of the intrinsic region 60 is large, and thus it takes a long time to supply carriers to the intrinsic region 60. Also, from the perspective of discharging the supplied carriers, in the P-I-N diode structure, an inverse bias should be applied to quickly discharge the carriers from the intrinsic region 60, and even when the carriers are discharged by applying an inverse bias, it takes a long time to discharge the carriers because the life time of the minority carriers in the intrinsic region 60 is long.
Accordingly, since the time for supply and discharge of the carriers is long, the variation of the refractive index of the ring optical waveguide is slow and thus the modulation speed of the optical modulator is also limited. Thus, the modulation speed of a ring optical modulator using a ring resonator having a P-I-N structure is not greater than 10 Gbps presently. Also, the current required for supplying and discharging carriers to and from an intrinsic region of the P-I-N structure is also considerably large.